Cellular telephones are known to utilize a tone signal to indicate the beginning of a TDM-Frame as well as to transmit data. The tone signal is generally of a sinusoidal nature which can be readily detected by a tone detection circuit. FIG. 1 illustrates a prior art toned detection circuit that includes a sampler 101, an analog-to-digital converter 103, a high pass filter 104, and adaptive band pass filter 105, a pole adapter 106, a first and second energy estimators 107-108, a gain adapter 109, and a tone detection counter 110. In operation, the sampler 101 receives a signal 100 and demodulates it in to an in-phase signal component 102. The in-phase signal component 102 is then sampled by the A-D converter 103 at four times the input signal rates. The resulting sampled signal is then passed through a high pass filter 104 and inputted to an adaptive band-pass filter 105.
The adaptive band-pass filter 105 is controlled by the gain adapter 109 based on an energy comparison between the energy of the input signal to the adaptive band-pass filter 105 and the energy of the output signal of the adaptive band-pass filter 105 as determined by the energy estimators 107-108. This energy comparison is performed approximately 160 times, wherein for each of these comparisons, the center frequency of the adaptive band-pass filter 105 is adjusted. When the energy estimator 107 and energy estimator 108 sense approximately the same energy, the frequency of the input signal 100 is determined.
Once the input signal frequency is determined, the tone detection counter 110 is incremented. The tone detection count is used to verify that an actual tone signal is being received, in that, a tone signal is several cycles, where each cycle can be counted.
While the above described tone detection circuit accurately detects when a cellular telephone is receiving a tone signal, the implementation is quite comprehensive. In practice, this type of tone circuit is implemented in a dedicated digital signal processor (DSP) or a microprocessor (.mu.P). To detect a tone signal, the DSP, or .mu.P, executes 14,400 cycles, 90 cycles for each of the 160 energy comparisons. Because of the large number of execution cycles needed to detect a tone signal, the DSP, or .mu.P, consumes a considerable amount of current, i.e., in the order of 5-20 mA.
In cellular telephones, the customer driven technological trend is to extend the battery life by reducing current drain requirements. Because of the current requirements of the above mentioned prior art tone detection circuit, cellular telephone battery life is not optimized. Therefore, a need exists for a tone detection circuit that reduces the number of cycle steps required to determine tone detection, thus reducing power consumption and reducing the cost for the DSP, or .mu.P, because a less powerful device can be used.